The present applicant's international patent publication WO91/03747 (application PCT /AU90/00395) proposes a diffraction grating structure comprised of a multiplicity of pixels which are individual optical diffraction gratings so that the pixellated diffraction grating when illuminated generates an optically variable image. The applicant's pixellated diffraction gratings utilising curved line grating pixels have become known by the trademark Pixelgram (trade mark). According to preferred aspects of the arrangement disclosed in the international application, the respective diffraction grating of each grating pixel comprises a plurality of reflective or transmissive grooves or lines which are usually curved across a pixel. Groove or line curvature determines both local image intensity, e.g. shading, and local optical structural stability. Groove or line spacing in each pixel determines local colour properties, with non-primary colours generated by a pixel mixing. Average groove or line orientation determines movement or colour effects, and the number of distinct values of average curvature and average spacing may be viewed as defining the Pixelgram palette, by analogy with the language of computer graphics. A further disclosure of a security diffraction grating structure is to be found in international patent publication WO90/07133 (PCT/AU89/00542).
The present applicant's international patent publication WO93/18419 (PCT/AU93/00102) discloses how selected visually observable effects in the optically variable image may be generated by arraying the pixels in groups -within which the pixels are arranged according to a predetermined rule for the pixellated diffraction grating. Thus, for example, multiple sets of different images, or of the same image but different shading or colour, may be produced at different viewing angles.
The concept of providing multiple optically variable images at different viewing angles, using a pixellated diffractive device in which each pixel contains a sub-pixel corresponding to each image, is also disclosed in U.S. Pat. No. 5,032,003. In that case, each diffractive sub-pixel is a straight line grating. This is an example of a more general class of pixellated diffractive structures utilising straight line grating pixels and known by the trademark Kinegram.
Australian patent application 10499/92 proposes a pixellated diffraction grating structure with three channels which constitute views from different angles of the same image, in order to obtain a stereoscopic image. The gratings may be curved line gratings. A predecessor of this reference is Japanese patent (Kokai) publication 2-72320.
European patent publication 467601 is concerned with holographic diffraction grating patterns which may include curved line gratings. Overlaid or alternate channels are proposed for providing different images at different angles. The different images may include numerical information and logos.